1. Field of the Invention
The present invention relates to an intelligent welding gun provided with a sensor, and various kinds of methods conducted by using the welding gun including a calibration method of a sensor, a control method of welding (which includes, for example, a control method of a pressing force, a feedback control method of re-welding, a control method of welding strength, a control method of suppressing the generation of a weld expulsion, and a control method of a track of a welding robot), and a managing method of a change in a positional accuracy of a welding point.
2. Description of Related Art
Japanese Patent Publication No. 10-94882 discloses a method for controlling a pressing force of a welding gun. In the method, a small quantity of an elastic displacement, generated in a fixed side electrode tip when a moving side electrode tip is driven to contact the fixed side electrode tip and further driven to press the fixed side electrode tip, is detected by an encoder of a servo motor for driving the moving side electrode tip. More particularly, the elastic displacement is determined based on the rotation increment of the servo motor generated from the time when the moving side electrode tip begins to contact the fixed side electrode tip to when the electric current suddenly increases when the moving side electrode tip is further driven to press the fixed side electrode tip. A real pressing force between the electrode tips is calculated based on the measured elastic displacement of the fixed side electrode tip. Then, a setting pressing force between the electrode tips is modified to be equal to the calculated real pressing force.
However, there are the following problems with the above-described conventional method for controlling a welding gun.
First, since a moving side portion of the welding gun including the moving side electrode tip and the servo motor for driving the moving side electrode tip is provided with a speed reducer, a mechanical impedance of the moving side portion is greater and dynamically harder than that of a fixed side portion of the welding gun. In this instance, the mechanical impedance is defined as an impedance expressed by a vector of |m, c, k|, when a movement of the electrode tip is expressed by an equation:
mxc2x7d2x/dt2+cxc2x7dx/dt+kx=F (t). 
In a case where the vector only includes k, the mechanical impedance is a spring constant. The encoder is located on the opposite side of the moving side electrode tip with respect to the speed reducer of the servo motor, so that a pressing quantity of the moving side electrode tip, transmission of a change in the pressing displacement and the pressing force to the encoder through the speed reducer is a small amount and is delayed. As a result, the responsibility is lowered, so that it is difficult to obtain an accurate response with the conventional method, in which the welding gun is controlled based on an output of the encoder.
Second, since the rigidity of an arm supporting the fixed side electrode tip is increased so that the mechanical impedance of the fixed side portion including the fixed side electrode tip and the arm is nearly equal to the mechanical impedance of the moving side portion, the size and the weight of the arm is large, which causes the welding gun to be increased both in size and weight.
An object of the present invention is to provide a welding gun which enables control with a high response and accuracy, and to provide various kinds of control methods conducted using the welding gun.
Another object of the present invention is to provide a welding gun which allows an arm supporting a fixed side welding tip to be decreased in size and rigidity, and to provide various kinds of control methods conducted using the welding gun.
The present invention for achieving the above objects are, as follows:
A welding gun includes a moving side portion including a moving side welding tip and a driving device for driving the moving side electrode tip, and a fixed side portion including a fixed side welding tip and an arm supporting the fixed side welding tip. A fixed side sensor for detecting at least one of a position of the fixed side welding tip and a pressing force imposed on the fixed side welding tip is provided in the fixed side portion.
In the welding gun, a mechanical impedance of the fixed side portion is smaller than that of the moving side portion. The mechanical impedance of the fixed side portion is set in a range where the fixed side sensor can effectively detect the at least one of the position of the fixed side welding tip and the pressing force imposed on the fixed side welding tip.
The fixed side sensor is any one of a force sensor, an optical distance sensor and a sensor using an optical fiber.
A moving side sensor for detecting at least one of a position of the moving side welding tip and a pressing force caused in the moving side welding tip may be provided in the moving side portion. In that case, the fixed side sensor and the moving side sensor constitute a redundant sensor measurement system.
A first method, which is a method of calibrating a sensor conducted using the welding gun, includes: (a) releasing the moving side welding tip from a position of contact with the fixed side welding tip; and (b) calibrating a reference point of at least one of a pressing force information and a positional information of the fixed side sensor.
A second method, which is a method of calibrating a sensor conducted using the welding gun, includes: (a) increasing pressure of the moving side welding tip against the fixed side welding tip and plotting a pressing force and/or a positional information to obtain characteristic curves of the moving side sensor and the fixed side sensor using a method of least squares; and (b) determining gains of the sensors such that the gains of the moving side sensor and the fixed side sensor are equal to each other.
A third method, which is a control method of welding a workpiece conducted using the welding gun, includes: (a) determining whether an expansion quantity of a welding portion of the workpiece detected by the fixed side sensor is equal to or greater than a predetermined value, and ending welding of the current welding point when the expansion quantity is equal to or greater than the predetermined value; (b) increasing the welding electric current when the expansion quantity is smaller than the predetermined value; (c) counting the number of times the welding electric current is increased and determining whether a re-welding program should be conducted when the counted number of times exceeds a predetermined number, and ending welding at the current welding point when it is determined that the re-welding program should not be conducted; (d) conducting re-welding when it is determined that the re-welding program should be conducted; and (e) determining, when re-welding is conducted, whether the expansion quantity of the welding portion of the workpiece during re-welding is equal to or greater than the predetermined value, ending welding of the current welding point when the expansion quantity is equal to or greater than the predetermined value, while issuing a warning when the expansion quantity does not reach the predetermined value.
A fourth method, which is a control method of welding a workpiece conducted using the welding gun, includes: (a) obtaining an expansion quantity of a welding portion of the workpiece, a position and a pressing force of the fixed side welding tip, from detected values detected at every moment by the fixed side sensor; (b) determining whether an expulsion is generated in the welding portion by comparing at least one of a value of the pressing force and the position of the fixed side welding tip at a point when the expansion quantity begins to decrease with at least one of a value of the pressing force and the position of the fixed side welding tip after a predetermined period of time has passed from the beginning of the decrease in the expansion quantity; and (c) setting a welding electric current of a corresponding welding point in a next cycle to be equal to or greater than a welding electric current of the current welding point when expulsion is not generated, while setting the welding electric current of the corresponding welding point in the next cycle to be smaller than the welding electric current of the current welding point when the expulsion is generated, thereby reflecting the data of the current cycle on a welding condition of the next cycle.
A fifth method, which is a method of managing a welding quality conducted using the welding gun, includes storing data about an expansion quantity, information about whether re-welding has been conducted and information about whether expulsion has been generated into a memory at each welding point after welding of the each welding point has been conducted, and periodically storing the data into a managing system of a higher level.
A sixth method, which is a control method of welding a workpiece conducted using the welding gun, includes: (a) obtaining an expansion quantity of a welding portion of the workpiece, a position of the fixed side welding tip, a differential value of the position, a pressing force, and a differential value of the pressing force which change at every moment from detected values detected at every moment by the fixed side sensor; (b) determining whether a sign of an expulsion generation exists in the welding portion by comparing at least one of the differential value of the position of the fixed side welding tip and the differential value of the pressing force from the beginning of a decrease in the expansion quantity with a predetermined value, at every moment; and (c) decreasing or stopping the welding electric current, and/or, reducing the pressing force when the sign of the expulsion generation exists, thereby reflecting on the welding electric current and/or the pressing force in realtime.
A seventh method, which is a control method of welding a workpiece conducted using the welding gun, includes: (a) determining whether an expansion quantity of a welding portion of the workpiece detected by the fixed side sensor is equal to or greater than a predetermined value, and ending welding of the current welding point when the expansion quantity is determined to be equal to or greater than the predetermined value; (b) increasing a welding electric current when the expansion quantity is smaller than the predetermined value; and (c) decreasing the welding electric current when a decrease in the expansion quantity is found during welding.
An eighth method, which is a control method of a pressing force of welding conducted using the welding gun, includes: (a) detecting contacting positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip, respectively, with a workpiece when the moving side welding tip and the fixed welding tip begin to contact the workpiece; (b) calculating differentials between objective positions xT and xTxe2x80x2 of the moving side welding tip and the fixed side welding tip, respectively, which are previously stored in a welding robot, and the contacting positions x1 and x1xe2x80x2, respectively; and (c) continuing pressing the workpiece, when differentials exist between the objective positions xT and xTxe2x80x2 and the contacting positions x1 and x1xe2x80x2, respectively, based on gains proportional to the differentials until the differentials become zero.
A ninth method, which is a control method of a pressing force of welding conducted using the welding gun, includes: (a) detecting contacting positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip with a workpiece, respectively, when the moving side welding tip and the fixed welding tip begin to contact the workpiece; (b) calculating differentials between objective positions xT and xTxe2x80x2 of the moving side welding tip and the fixed side welding tip, respectively, which are previously stored in the welding robot, and the contacting positions x1 and x1xe2x80x2, respectively; (c) continuing pressing the workpiece, when differentials exist between the objective positions xT and xTxe2x80x2 and the contacting positions x1 and x1xe2x80x2, respectively, based on gains proportional to the differentials and returning to the step of calculating the differentials, while obtaining arriving positions x2 and x2xe2x80x2 of the moving side welding tip and the fixed side welding tip, respectively, from the current detected positions of the moving side welding tip and the fixed side welding tip when differentials do not exist; (d) calculating a pressing force P0 required for the moving side welding tip and the fixed side welding tip to reach the arriving positions; (e) adding a pressing force P1 necessary for welding to the pressing force P0 and imposing the total pressing force PT which is a summation of the P0 and P1 on the workpiece; and (f) pressing a welding electric current between the moving side welding tip and the fixed side welding tip thereby conducting welding.
A tenth method, which is a method of correcting a track of a welding robot conducted using the welding gun, includes: (a) detecting contacting positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip with a workpiece, respectively, when the moving side welding tip and the fixed welding tip begin to contact the workpiece; (b) calculating differentials between objective positions xT and xTxe2x80x2 of the moving side welding tip and the fixed side welding tip, respectively, which are previously stored in the welding robot, and the contact positions x1 and x1xe2x80x2, respectively; (c) continuing pressing the workpiece, when differentials exist between the objective positions xT and xTxe2x80x2 and the contact positions x1 and x1xe2x80x2, respectively, based on gains proportional to the differentials and returning to the step of calculating the differentials, while obtaining arriving positions x2 and x2xe2x80x2 of the moving side welding tip and the fixed side welding tip, respectively, from current detected positions of the moving side welding tip and the fixed side welding tip, when differentials do not exist; and (d) correcting the objective positions so that the differentials between the objective positions xT and xTxe2x80x2 and the arrival positions x2 and x2xe2x80x2, respectively, become zero.
An eleventh method, which is a method of managing a change in a positional accuracy of a welding point conducted using the welding gun, includes: (a) entering positional information x1, x1xe2x80x2, x2 and x2xe2x80x2 from a database storing contacting positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip with a workpiece at a time when the moving side welding tip and the fixed side welding tip begin to contact the workpiece, and arriving positions x2 and x2xe2x80x2 of the moving side welding tip and the fixed side welding tip at a time when the moving side welding tip and the fixed side welding tip have pressed the workpiece; (b) calculating a positional accuracy vector of a station with respect to a plurality of stations each having at least one robot, the matrix being defined by the following:
|"PHgr"n|=[|P1|, |P2|, . . . , |Pm|]
wherein,
n: a station number of the current station
m: the number of robots equal to or greater than 1, of station n
|Pj|: a positional accuracy matrix of a robot (No. j robot), obtained from the welding points P1, P2, . . . , and Pk and positions x1, x1xe2x80x2, x2, and x2xe2x80x2 of the robot;
and (c) managing a change in a positional accuracy of the welding point of the workpiece based on a value and/or values: |"PHgr"n|xe2x88x92|"PHgr"nxe2x88x921| and/or |"PHgr"n|xe2x88x92|"PHgr"1|.
With the above welding gun, since the sensor is provided in the fixed side portion, the sensor can be disposed in a position where the mechanical impedance is smaller than that of the moving side portion and where is not disposed via a gear such as a speed reducer from the welding tip. As a result, a displacement of the welding tip and a pressing force imposed on the welding tip can be detected with high accuracy and a good response. By controlling the welding gun according to the output of the sensor, a scope of objects capable of being controlled is widened and the welding gun can be made intelligent.
With the above welding gun, since the mechanical impedance of the fixed side portion is set in a range where the sensor can effectively detect a displacement of the fixed side welding tip and the pressing force, the mechanical impedance of the fixed side portion can remain small, unlike a conventional welding gun in which the mechanical impedance of the fixed side portion is increased to be nearly equal to a mechanical impedance of the moving side portion. As a result, the arm supporting the fixed side welding tip can be decreased both in rigidity and in size as compared with the conventional welding gun, thereby making the welding gun compact and lightweight. Since the fixed side portion is not provided with a speed reducer gear or the like, the mechanical impedance of the fixed side portion is necessarily smaller than that of the moving side portion. By utilizing the small mechanical impedance as it is, the displacement of the fixed side welding tip when pressed is made large. Thus, sensitive and accurate detection is made possible, which also makes the welding gun intelligent.
With the above welding gun, since the fixed side sensor is any one of a force sensor (a load sensor), a distance sensor (a displacement sensor) and a sensor using an optical fiber, a commercial sensor can be used.
In the case where sensors are provided both in the fixed side portion and the moving side portion of the welding gun, the fixed side sensor and the moving side sensor can constitute a redundant sensor measurement system. Therefore, by using one sensor, calibration of a reference point, confirmation of a normal operation, etc. of the other sensor can be performed.
In the first method that is a method of calibrating a sensor conducted using the welding gun, since the reference point of the fixed side sensor is calibrated in a state that the moving side welding tip is released from the fixed side welding tip, the reference point of the fixed side sensor can be calibrated based on an output from the moving side sensor.
In the second method that is a method of calibrating a sensor conducted using the welding gun, since the moving side welding tip is pressed against the fixed side welding tip and the gains of the moving side sensor and the fixed side sensor are adjusted, one sensor can calibrate the other sensor.
In the third method that is a method of controlling welding conducted using the welding gun, the method can be conducted without using the moving side sensor. Also, the method can be conducted even if the driving device for the moving side portion is not a servo motor but an air cylinder. Further, a re-welding feedback control can be performed. When the expansion quantity of the welding portion during welding does not reach the predetermined value even though the welding electric current is increased which may be caused by dust or the like adhering to the workpiece or by a malfunction of the apparatus re-welding is conducted, because it may work in the case of the adhesive dust or the like. If the expansion quantity of the welding portion still does not reach the predetermined value despite re-welding, it is determined that the apparatus has malfunctioned, and a warning is issued. The warning may be substituted with stoppage.
In the fourth method that is a method of controlling welding conducted using the welding gun, the method can be conducted without using the moving side sensor. Also, the method can be conducted even if the driving device for the moving side portion is not a servo motor but an air cylinder. Further, generation of the expulsion in the corresponding welding point in the next cycle can be suppressed. In a normal condition, the expansion quantity (or the pressing force) of the welding portion during welding generally increases and forms a positive exponential curve (et) and decreases and forms a negative exponential curve (exe2x88x92t) after reaching a peak (where the welding electric current stops). In contrast, when the expulsion is generated, the expansion quantity (or the pressing force) suddenly decreases simultaneously with the expulsion generation and then returns to a value smaller than that at the beginning of welding. When the expansion quantity is continuously detected and it is detected that the expansion quantity suddenly decreases as compared with the predetermined curve of the normal condition, it can be determined that the expulsion has just been generated, and the welding electric current of the corresponding welding point in the next cycle should be decreased. As a result, generation of the expulsion in the corresponding welding point in the next cycle can be suppressed. With respect to control of the welding gun, since a very small change in the expansion quantity has to be detected, it is difficult in the detected quantity and responsibility to perform an accurate control of the expansion quantity by using the conventional servo motor encoder. By using a welding gun provided with the fixed side sensor which can sense data on the order of every 10xe2x88x926 seconds, an accurate detection and the aforementioned control are possible.
In the fifth method that is a method of managing a welding quality conducted using the welding gun, since data about the expansion quantity, information about whether re-welding has been performed and information about whether expulsion has been generated are stored into the memory at every welding point in the third and the fourth methods, and are periodically stored into the managing system of a higher level, welding quality can be managed.
In the sixth method that is a method of controlling welding conducted using the welding gun, the method can be conducted without using the moving side sensor. Also, the method can be conducted even if the driving device for the moving side portion is not a servo motor but an air cylinder. Further, generation of the expulsion in the current welding point can be suppressed. In a normal condition, the expansion quantity (or, the pressing force) of the welding portion during welding generally increases and forms a positive exponential curve (et) and decreases and forms a negative exponential curve (exe2x88x92t) after reaching the peak (where the welding electric current stops). In contrast, when the expulsion is generated, a gradient of the curve of the expansion quantity (or the pressing force) begins to decrease, then rapidly decreases and returns to a value smaller than that at the beginning of welding. When the expulsion is detected at every moment during welding and the gradient of the curve is then calculated at every moment and it is found that the gradient decreases more greatly than a predetermined allowable value, it is determined that a sign of the expulsion generation exists and the welding electric current is controlled to be decreased. As a result, generation of an expulsion in the current welding point can be suppressed. In the control, since a very small change in the gradient has to be detected, it is difficult in quantity and responsibility to perform an accurate control of the expansion quantity by using the conventional servo motor encoder. By using the welding gun provided with the fixed side sensor which can perform a control of the order of 10xe2x88x926 seconds, an accurate detection and the aforementioned control are possible.
In the seventh method that is a method of controlling welding conducted using the welding gun, the method can be conducted without using the moving side sensor. Also, the method can also be conducted even if the driving device for the moving side portion is not a servo motor but an air cylinder. Further, in the seventh control method, the welding strength can be controlled. In general, there is a correlation between a welding electric current, and a nugget size and a thermal expansion quantity. In addition, there is a correlation between the nugget size and the thermal expansion quantity, and a welding strength. It is generally considered that the spot welding has a sufficient welding strength when the thermal expansion quantity reaches the predetermined value. Therefore, the welding electric current is increased until the thermal expansion quantity reaches the predetermined value. However, when the thermal expansion quantity decreases during pressing, which means that some welding expulsion is generated, the welding electric current is decreased. By repeating this routine during welding, spot welding having a necessary welding strength can be conducted in a minimum time period under a condition that expulsion is not generated.
In the eighth method that is a method of controlling welding conducted using the welding gun, both the fixed side sensor and the moving side sensor are used. The driving device for the moving side portion may be a servo motor or an air cylinder. In the eighth method, the control for the pressing force is conducted based on information about positions x1, x1xe2x80x2 of the moving side welding tip and the fixed side welding tip at a time when the welding tips reach the contacting points with a workpiece (at that time the electric current of the motor for diving the moving side welding tip suddenly increases). More particularly, differentials between the objective positions xT and xTxe2x80x2 of the moving side welding tip and the fixed side welding tip and the contacting positions x1 and x1xe2x80x2 are calculated, respectively. When some differentials exist, which means that the workpieces have a clearance therebetween, the pressing force continues to be imposed on the workpiece based on gains proportional to the differentials. Due to this pressing, the workpieces are pressed and the clearance between the workpieces is eliminated. When the differentials become zero or the differentials do not exist, the arriving positions x2 and x2xe2x80x2 of the moving side welding tip and the fixed side welding tip are obtained from the currently detected positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip. Due to this operation, even if a clearance exists between the workpieces, the clearance is eliminated and then welding is conducted, so that a spot welding of a high quality causing no separation can be performed.
In the ninth method that is a method of controlling a pressing force of welding conducted using the welding gun, both the fixed side sensor and the moving side sensor are used. The driving device for the moving side portion may be a servo motor or may be an air cylinder. In the ninth method, controlling the pressing force is conducted based on information about positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip when the welding tips reach the contacting points with a workpiece (at that time the electric current of the motor for driving the moving side welding tip suddenly increases). More particularly, the differentials between the objective positions xT and xTxe2x80x2 of the moving side welding tip and the fixed side welding tip and the contacting positions x1 and x1xe2x80x2 are calculated, respectively. When differentials exist, which means that the workpieces have a clearance therebetween, the pressing force continues to be imposed on the workpiece based on gains proportional to the differentials. Due to this pressing, the workpieces are pressed and the clearance between the workpieces is eliminated. When the differentials becomes zero or the differentials do not exist, the arriving positions x2 and x2xe2x80x2 of the moving side welding tip and the fixed side welding tip are obtained from the currently detected positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip. Then, the pressing force P0 required for the welding tips to reach the respective arriving points is calculated. The pressing force P0 is a force imposed on the workpieces in order to eliminate the clearance between the workpieces and therefore is a spring back force of the workpieces. A pressing force P1 necessary for welding is further added to the pressing force P0, and the total pressing force P0 of P1 and PT is imposed on the workpieces. Due to this operation, even if a clearance exists between the workpieces, the pressing force P1 necessary for welding can be imposed on the workpieces, so that a spot welding of a high quality having a sufficient pressing force can be performed.
In the tenth method that is a method of correcting a track of a welding robot conducted using the welding gun, both the fixed side sensor and the moving side sensor are used. The driving device for the moving side portion may be a servo motor or may be an air cylinder. In the tenth method, controlling the pressing force is conducted based on information about positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip when the welding tips reach the contacting points with a workpiece (at that time the electrical current of the motor for driving the moving side welding tip suddenly increases). More particularly, the differentials between the objective positions xT and xTxe2x80x2 of the moving side welding tip and the fixed side welding tip and the contacting positions x1 and x1xe2x80x2 are calculated, respectively. When differentials exist, which means that the workpieces have a clearance therebetween, the pressing force continues to be imposed on the workpieces based on gains proportional to the differential. Due to this pressing, the workpieces are pressed and the clearance between the workpieces is eliminated. When the differentials become zero or the differentials do not exist, the arriving positions x2 and x2xe2x80x2 of the moving side welding tip and the fixed side welding tip are obtained from the currently detected positions x1 and x1xe2x80x2 of the moving side welding tip and the fixed side welding tip. The arriving positions x2 and x2xe2x80x2 are real arriving positions. By correcting the objective positions xT and xTxe2x80x2 to the real positions x2 and x2xe2x80x2, the track of the welding robot is modified and is prepared for welding of the corresponding welding point of the next cycle.
In the eleventh method that is a method of managing a change in a positional accuracy of a welding point conducted using the welding gun, both the fixed side sensor and the moving side sensor are used. The driving device for the moving side portion may be a servo motor or may be an air cylinder. In the eleventh method, the positional accuracy matrix of each station |"PHgr"n| is calculated from the contacting positions and pressing positions at the respective welding points and then the change in the positional accuracy at the welding point of the workpiece is managed based on the value and/or values of |"PHgr"n|xe2x88x92|"PHgr"nxe2x88x921| and/or |"PHgr"n|xe2x88x92|"PHgr"1|. Therefore, a deformation of the workpiece due to welding can be suppressed, for example, by changing the welding order of the welding points thereby obtaining an optimum welding order in which that the value of |"PHgr"n|xe2x88x92|"PHgr"nxe2x88x921| is further decreased.